responseMimeType.application/json is incompatible with function-calling modes in ADK agent config

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responseMimeType.application/json is incompatible with function-calling modes in ADK agent config

🔴 Required Information

Describe the Bug:
ADK Agent Config accepts a configuration where responseMimeType is set to application/json while tools are enabled and functionCallingConfig.mode is set to AUTO or ANY . Although responseMimeType: application/json is documented as a supported option, this configuration causes the Gemini backend to reject the request with a 400 INVALID_ARGUMENT error.

However, when functionCallingConfig.mode is set to NONE, the agent runs successfully and produces the expected output.

Steps to Reproduce:
Please provide a numbered list of steps to reproduce the behavior:

1. Install google-adk.
2. Create an ADK Agent Config with tools enabled.
3. Set generate_content_config.responseMimeType to application/json.
4. Set generate_content_config.toolConfig.functionCallingConfig.mode to AUTO.
5. Run the agent with gemini-2.5-pro.
6. Observe that the request fails with 400 INVALID_ARGUMENT error.

Expected Behavior:
A clear and concise description of what you expected to happen.

1. Reject this incompatible configuration during configuration loading
2. Clearly document this limitation in the ADK Agent Config documentation.

Observed Behavior:
error message

google.genai.errors.ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': "Function calling with a response mime type: 'application/json' is unsupported", 'status': 'INVALID_ARGUMENT'}}

Environment Details:

• ADK Library Version (pip show google-adk): 1.33.0
• Desktop OS:** [e.g., macOS, Linux, Windows] Windows
• Python Version (python -V): 3.11.15

Model Information:

• Are you using LiteLLM: No
• Which model is being used: (e.g., gemini-2.5-pro) gemini-2.5-pro

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🟡 Optional Information

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Logs:
Please attach relevant logs. Wrap them in code blocks (```) or attach a
text file.

google.genai.errors.ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': "Function calling with a response mime type: 'application/json' is unsupported", 'status': 'INVALID_ARGUMENT'}}

Screenshots / Video:
If applicable, add screenshots or screen recordings to help explain
your problem.

Additional Context:

Minimal Reproduction Code:
agent config root_agent.yaml

generate_content_config:
  temperature: 0.1
  maxOutputTokens: 4000
  responseMimeType: application/json
  toolConfig:
    functionCallingConfig:
      mode: AUTO

How often has this issue occurred?:

• Always (100%)